High and low temperature damp heat test chambers are commonly used in instruments, materials, and electronic products, mainly to test various performance indicators in high and low temperature or damp heat environments. In practical operation, the high and low temperature wet heat test chamber will perform humidification and dehumidification operations to achieve the test conditions. You probably already know the humidification method, so how to dehumidify?
The definition of humidity refers to the ratio of the partial pressure of water vapor in the air to the saturated vapor pressure of water at that temperature, expressed as a percentage. From the property of water vapor saturation pressure, it can be seen that the saturated pressure of water vapor is only a function of temperature and is independent of the air pressure at which water vapor can exist. People have sought to express the relationship between water vapor saturation pressure and temperature through a large number of experiments and organization. Among them, the Gough's formula has been widely used in engineering and metrology, and customers often mention the humidity range of 0-100% RH， This is actually an incorrect range value. 100% humidity can no longer be called humidity because humidity refers to water mist and vapor, and 100% humidity has already formed water droplets. 0% humidity is already used to treat dry environments (such as high temperatures above 100 degrees and low temperatures below 0 degrees). Therefore, the correct humidity range should be 30-98% R.h. If low humidity is desired, it can be reduced to 10% or 20%.

The dehumidification principle of high and low temperature test chambers: During the operation of the refrigeration system, the temperature of the fins on the surface of the evaporator will be lower than the dew point temperature of the test temperature. During air circulation, moisture will condense when passing through the surface of the evaporator, thus achieving the purpose of dehumidification.

Principle of wet heat alternation in high and low temperature test chambers:

When the air in the test chamber passes through an open water surface, it undergoes heat and moisture exchange with the water surface. Depending on the water temperature, only sensible heat exchange may occur, or there may be both sensible heat exchange and moisture exchange, as well as latent heat exchange. When the air in the test chamber comes into direct contact with the water surface, a saturated air boundary layer with a temperature equal to the water surface temperature is formed near the water surface due to the irregular motion of water molecules, and the concentration or partial pressure of water vapor molecules depends on the saturated air temperature of the boundary layer.

If the concentration of water vapor molecules in the test chamber is greater than that in the air above it (i.e. the partial pressure of water vapor in the boundary layer is greater than that in the air), the number of water vapor molecules in the air will increase; Conversely, it will decrease. The former is called "evaporation", while the latter is called "condensation". During the evaporation process, the reduced water vapor molecules in the boundary layer are supplemented by water molecules that jump out of the water surface. During the condensation process, excessive water vapor molecules in the boundary layer will return to the water surface. Therefore, the sensible heat exchange between air and water depends on the temperature difference between the boundary layer and the air above it, while the wet exchange and the resulting latent heat exchange depend on the concentration or partial pressure difference of water vapor molecules between the two.

There are currently two methods of dehumidification used, one is the freezing dehumidification method, and the other is the solid desiccant dehumidification method. The former condenses water vapor in the air onto the surface cooler to form water or frost. Due to the long testing process of the wet heat chamber, frosting on the surface cooler can affect the dehumidification effect. Generally, this phenomenon should be avoided as much as possible. In order to prevent frosting on the surface cooler, the temperature of the surface cooler should be controlled above 0 ℃. When describing the humidity inside the box as dew point, the dew point temperature is approximately 5-7 ℃. This dew point temperature can meet the requirements of current testing methods and is very convenient to use, so it is widely used. When a lower dew point is required, solid hygroscopic agents are usually used to further absorb moisture. The surface water vapor pressure of this type of moisture absorbent is on the order of several hundred to tens of PPm, and can achieve a dew point temperature of around -70 ℃. This method is very inconvenient to use or it is very expensive to purchase specialized equipment. It is only adopted in some experiments with special requirements. When the internal combustion engine is in a low-temperature or low-temperature chamber for operation testing, it is required to supplement a large amount of air inside the chamber for fuel combustion. To prevent a large amount of frost on the evaporator of the low-temperature box caused by water vapor in the new air from affecting refrigeration, a rotary dehumidifier made of solid moisture absorption principle and capable of continuous operation is needed. At present, the commodity price of this dehumidifier is very expensive.